For American high-energy physicists, the Superconducting Supercollider (SSC) accelerator to be built near Dallas is the only game in town for the kind of physics they now want to explore. That's the physics that occur when particles interact at energies of 20 trillion electron volts (teV). Those are energy conditions that prevailed a few moments after our universe's birth. This is 20 times the energy that protons gain in the United States' most powerful operating accelerator - the Tevatron at Fermilab near Chicago.

But there is also consensus among these physicists that the SSC probably is the last of its species. It will be the ultimate achievement of available accelerator technology. Superconducting magnets will channel two counter-rotating particle beams around a circular vacuum tube while radio waves feed energy to the particles. The beams will collide to produce the phenomena under study.

This design pushes available technology so hard that, even several years after the project's approval, SSC cost estimates are uncertain and rising steeply. They have gone from an initial $4.5 billion in 1987 to a ``firm'' $5.9 billion in January 1989 to the $8 billion and still uncertain figure US Deputy Energy Secretary Henson Moore recently quoted. Among other uncertainties, engineers must redesign the magnets, and the main accelerator ring may be larger than the planned 54-mile circumference.

As Robert Siemann, a Cornell University physicist who helped arrive at the latest cost estimate, observes, ``The SSC is the only solution we know about to do this kind of physics in this time frame [late 1990s].'' But, he adds, ``It would be literally inconceivable to scale up this way of accelerating particles by 10-fold again.''